Nightmare monster

A nightmare monster is a unique type of monster that appears in the Sony PlayStation and Sega Saturn versions of Doom, and also re-appearing in the 2020 re-release of Doom 64 and in source ports such as Doom64 EX.

Description
In the PlayStation version by Williams Entertainment, any thing which can be placed on a map can be given the "nightmare" flag, and will become subtractively blended and double its spawn health. The Sega Saturn version keeps this behavior but is unable to render monsters differently from regular spectres.

In the official games, the only monster to ever have this flag applied to it is the spectre, resulting in the nightmare spectre appearing in several levels in the PlayStation and Saturn versions. While the flag can be used on any monster (or indeed, any thing, including items or decorations), this is never actually done in any of the standard levels of the game. Pre-release screenshots from the PlayStation version of Doom do however show that the flag was at one point used for imps, but all of these were removed by the time of the game's release. Custom levels made for the game have since made use of some of these monsters, which appear in projects such as PlayStation Doom: Master Edition.

The ability to turn monsters into nightmare versions was removed in the initial release of Doom 64. Instead, the nightmare imp is featured as a new monster, which is stronger than the regular imp. However, this monster is actually unique programmatically and not the result of applying a flag to regular imps in the maps, making it entirely separate from the rest of the nightmare monsters discussed in this article.

The nightmare flag proper was re-added to Doom 64 as part of the 2020 re-release of the game, itself inheriting it from Doom64 EX where it was originally implemented. This time, nightmare monsters appear in a translucent green color. In addition to having more hit points and speed, their attacks are also affected, with higher damage values and projectiles fired by these monsters also having a translucent green color and travelling much faster than their regular counterparts. As before, the effect can also be used on any thing in the game, including items and props, making them look different even though they otherwise behave the same as their regular counterparts. Nightmare monsters never appear in any of the official levels in Doom 64 (including The Lost Levels), but they have become a staple of user-made PWADs for the game.